This invention relates to embedded servo data on magnetic discs, and particularly to a disc having irregularities of magnetic properties forming an embedded servo pattern, and a process for making and using the same.
In magnetic disc drives, data bits recorded on concentric tracks of magnetic discs are established by the orientation of magnetic dipoles in the disc medium. As the disc rotates, a confronting read head senses changes in the magnetic field due to changes in dipole orientation of data bits passing the head. Data may be in the form of user data and servo data. Servo data are used by the disc drive to control operations of the drive, such as head position, indexing, identification and track and sector addressing. In embedded servo systems, servo data are recorded on the same disc surface as user data; the servo data appearing in servo sectors located in pre-defined spokes spaced at regular intervals along the tracks. In contrast to embedded servo systems, dedicated servo systems employ a separate disc surface dedicated to the servo data. While the present invention is useful with dedicated servo systems, it will be described in conjunction with embedded servo systems.
Servo data are ordinarily recorded at a lower frequency than are user data. Presently user data are recorded at frequencies above about 500 Megahertz (MHz) and in some cases into the Gigahertz (GHz) range, whereas servo data are usually recorded at lower frequencies (30 to 120MHz).
The areal density of data recorded on a disc is limited by the amount of data that may be recorded on a single track and the number of concentric tracks that may be compacted onto the disc (track density). The track density is limited by track width and spacing, by the width of the read/write head confronting the disc surface, by the flying characteristics of the head, and by the positioning of servo data. There are problems in writing magnetic servo data for high density discs, particularly in the replication of the process for mass production of the discs. Consequently, interest has arisen in forming servo patterns with irregularities in the magnetic properties of the magnetic disc. These irregularities are usually in the form of pits or grooves in the recording surface. The pits or grooves are of sufficient depth as to cause a discontinuity in the magnetic field sensed by the read head. The pattern of discontinuities represents servo data.
One problem with the use of surface irregularities, such pits and grooves, is that the pits or grooves were of such a depth as to have a limiting effect on track density. More particularly, the pits or grooves needed to be deep enough to create magnetic field discontinuities so that the recovered servo data signal had a satisfactory signal-to-noise ratio. The deep pits and grooves caused difficulties with medium deposition and corrosion, and debris accumulated in the pits, often affecting the recorded servo data. While more shallow pits and grooves would permit fewer medium difficulties, shallow pits and grooves do not create magnetic field discontinuities that are necessary for satisfactory signal-to-noise ratios of the recovered servo signal. The present invention provides a solution to this and other problems, and offers other advantages over the prior art.
The present invention employs a disc medium having irregularities in magnetic properties in a pattern defining the servo pattern. A high frequency magnetic carrier signal is written through the servo pattern so that a read head passing by the servo pattern reads a varying magnetic field intensity to recover the servo data, such as by amplitude demodulation of the magnetic carrier. In one form of the invention, the irregularities are surface irregularities in the disc having elevations different from the elevation of the nominal read surface of the disc. In another form of the invention, the irregularities are due to altered magnetic properties of the medium.
In accordance with one embodiment of the present invention, a magnetic medium includes a read surface having a plurality of tracks. Surface irregularities in the read surface are in the form of pits or grooves that extend a defined depth into the body from the read surface along a servo portion of at least one of the plurality of tracks. The pits or grooves are arranged in a servo pattern that has a servo burst frequency. A magnetic carrier signal is recorded in the magnetic medium along the servo portion at a carrier frequency greater than the servo frequency.
In accordance with another embodiment of the magnetic medium, the magnetic coercivity of the disc is reduced in regions arranged in the servo pattern.
According to another aspect of the invention, a manufacturing process is provided by which a plurality of tracks are defined on the read surface of a magnetic disc. At least one servo sector portion is defined along each of at least some of the plurality of tracks, and first and second minimum read-back signal amplitudes to be produced by a confronting read head are defined to represent a first and second binary values. Irregularities are formed in the disc along each servo sector portion to define a desired servo pattern having a servo burst frequency. In one embodiment, the irregularities are pits in the disc surface; in another embodiment the irregularities are regions of reduced magnetic coercivity. A magnetic carrier signal is written into the magnetic disc along the servo sector portion at a carrier frequency greater than the servo burst frequency. In the embodiment employing pits, the pits have a depth, h, into the read surface of       h    =                  λ                  2          ⁢          π                    ⁢      ln      ⁢              xe2x80x83            ⁢              (                              V            2                                V            1                          )              ,
where V1 and V2 are the first and second signal amplitudes and xcex is the wavelength of the magnetic carrier signal.
According to one embodiment of the manufacturing process, the magnetic carrier signal is written into the disc by positioning a servo writer to confront a first radius of the read surface. The magnetic carrier signal is continuously written onto the disc along at least that much of the circumference at the first radius that includes a servo sector portion and a portion adjacent the servo sector portion. The servo writer is stepped to a second radius adjacent the first radius, and synchronized to the magnetic pattern written at the first radius. The magnetic carrier signal is continuously written onto the disc along at least that much of the circumference at the second radius that includes the servo sector portion and a portion adjacent the servo sector portion. The process is repeated until all of the servo sector portions are written with the magnetic carrier.
In one embodiment, the radial repositioning of the servo writer is in half-track increments.
According to another aspect of the present invention, the servo data are read from the magnetic disc by reading a varying magnetic field from a servo sector portion of a track on the disc to derive a read signal. The read signal is demodulated using a demodulating signal at the carrier frequency to derive a servo signal. The servo signal is recovered using a second signal at the servo burst frequency to derive servo data.